RFC: major runtime map changes

[Martin Hunt]

While we're talking about changes to maps, what should happen when an
_stp_map_set_xxx() fails due to lack of space? Currently this does not
happen because maps are set to wrap by default. So the oldest entry in
the map is cleared and reused. There is a BZ asking for that to be
changed.

So which of the following should happen?

1. wraps and returns an error code
2. fails (does not store value) and returns error code
3. increments a "drops" counter for the map
4. does stp_warn()